As disclosed in JP 06-40270A (JP 2567548), for example, an inter-vehicle distance control system, which controls a subject vehicle to maintain an inter-vehicle distance from a preceding vehicle at a target inter-vehicle distance based on a relative speed between the subject vehicle and the preceding vehicle.
According to this inter-vehicle distance control system, the time constant of a filter for processing a relative speed signal or information (data) is set small when the vehicle speed is high and the inter-vehicle distance is short, and the time constant of the filter is set large when the vehicle speed is low and the inter-vehicle distance is long. The inter-vehicle distance control system improves the control response characteristic by the small time constant and realizes the control stability.
Since the above inter-vehicle control system controls the speed of the subject vehicle to maintain the inter-vehicle distance at the target inter-vehicle distance, the acceleration/deceleration (acceleration and/or deceleration) of the vehicle will not necessarily match acceleration/deceleration, which a driver of the subject vehicle wants.
It is therefore proposed in US 2007/0021876 A1 (JP 2007-076632 A) that a vehicle drive assisting system is configured to accelerate and decelerate a subject vehicle based on a target relative acceleration/deceleration, which is calculated as a target between the subject vehicle and a preceding vehicle, based on a target driver condition coefficient and an actual driver condition coefficient. The target driver condition coefficient is a driver condition coefficient corresponding to a desired drive operation performed by a skilled or model driver to maintain an inter-vehicle distance between the subject vehicle and the preceding vehicle.
However, according to this proposed vehicle drive assisting system, both the target deceleration and the target acceleration are calculated based on only the inter-vehicle distance and the relative speed, and hence the acceleration and the deceleration are still not so satisfactory to a driver at some instances.
It is therefore further proposed in U.S. patent application Ser. No. 12/151,998 (JP patent application No. 2007-128568) filed on May 12, 2008 that a vehicle control system is configured to control acceleration based on a correction distance condition evaluation index KdBc, which is calculated in consideration of a travel speed of a preceding vehicle. This evaluation index indicates approach/separation (approach and/or separation) of the subject vehicle relative to the preceding vehicle. In this US patent application, it is indicated that braking start points (timings) are present on one characteristic curve when the evaluation index KdBc is calculated by the following equation Eq. 1. This is based on a result of testing a vehicle by starting to brake the vehicle at the latest possible timing not to crash the preceding vehicle under a condition that the subject vehicle is approaching the preceding vehicle. This characteristic curve is approximated as the following equation Eq. 2.KdBc=10×log{|(−2×|Vr|+α×|Vb|)/(D3×5×10−8)|}  (Eq. 1)KdBc=−23.76×log D+76.96   (Eq. 2)
For many drivers, use of the approximated equation Eq. 2 as the equation for determining the timing of starting the acceleration control and the deceleration control will provide generally satisfactory and comfortable starting of acceleration and deceleration. However, the timing of starting decelerating operation varies from driver to driver. As a result, the timing of starting the acceleration and deceleration determined by the approximated equation Eq. 2 will possibly be felt too early or too late.